Solar with AGM battery

[Natetheskate]

I have a 15 year old solar system in my class C: A single KC-50 panel with an older ASC 12/8 controller. The issue is even in bright sunlight, the system can't seem to get the battery above 12.7 volts, even if the array is putting out about 20 volts. I heard that the AGM batteries are not fully charged until around 13 volts. Do I need a different kind of controller? Any recommendations?

 

[tlmgcamp]

I understand that solar panels have a useful life of around 20 yrs. Since yours are 15 years old they are of an earlier vintage that may not be up to the current 20 year estimate

 

[Mark_K5LXP]

It's implied that if the panel voltage is sitting at open circuit voltage the controller isn't passing any current to the batteries. Could be because the controller thinks the batteries are full, or there's something else going on like an open fuse from the controller to the batteries. It "should" at least be at a trickle level (~13.4V) with illuminated panels even if the controller completed a charge cycle, so I would say at best there's something going on that bears investigation.

Mark B.
Albuquerque, NM

 

[John From Detroit]

If the panels are putting out 20 volts TO the controller but the batteries are not filling up..
I'd start by checking the controller. THEN the panels.. but as noted above panels are getting along in years.. Might be time to replace and the replacements may even be an Upgrade.

 

[LMHS]

My hybrid SLA/gel batteries (200AH 6V X 2) take the same charging as the LA batteries they replaced. I simply went to the new batteries website and got the charging info from there. If I'm looking at the right piece of paper (and I think I am) this is the input I programmed the cheapie PMW solar controller. At a guess, I would say to FIRST check to see what the specs are for your current battery. Change the numbers if needed. Check to see how OLD your batteries are. Most batteries need to be replaced after 5 to 7 years. Sooner if you murder them.

PMW Solar Controller
Float Voltage: 13.5v
Discharge Reconnect: 12.5v
Discharge stop: 11v
Work mode: 24H
Type of Battery: B01

Also bookmark and read this until you understand it. HandyBob's Blog

BTW, I use two Harbor Freight 100W mono solar panels. If there's a problem, I can take them back (got a 2yr warranty). I looked before I bought to make sure they weren't broken. On sale PLUS an employee discount meant they were slightly cheaper than online prices and I got them the same day. Installed the next day.

 

[Isaac-1]

Given that you can buy a decent quality 10 amp MPPT solar controller these days for under $30 (you can even get a top of the line Victron branded MPPT 10A solar controller online for under $45), I would start there (10A solar controller is good up to about a 120 watt panel), being MPPT it will greatly outperform your old shunt style solar controller, even a PWM controller will out perform your shunt type controller. So If it turns out the panel is bad, then replace it with a circa 100 watt panel, you will already have a good controller.

 

[Tom_M]

A couple of questions. First does the battery come up to full charge when plugged into shore power and how old is the battery?

 

[Mark_K5LXP]

A "bad" panel won't be putting out 20V. And while any panel can go open circuit, as they age their output power can start to diminish. At 20 years you might see a 5-10% reduction. Something here is open circuit, either a wire/fuse or the controller is TU. A quick test would be to connect the panels directly to the batteries and observe what happens to voltage and current.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

I heard that the AGM batteries are not fully charged until around 13 volts.

An AGM is still a lead-acid battery and like all lead-acid batteries it is fully charged at 12.6-12.7 volts, measured at rest with no current flowing in or out. To reach that state of charge requires a charging voltage of at least 13.0v and preferable 13.3v-14.3v.

That said, with your solar panel/controller connected and in full sun, I would expect the battery to reach 13.1v-13.6v. The panel voltage of 20 (did you verify that) isn't really relevant because the controller regulates (limiting if necessary) that for proper battery charging. You don't want 20v pushing amps into the battery under almost any circumstances.

I'd suggest isolating the batteries and measuring their open circuit voltage. And trying to charge them with another charger to see if they come up to rated voltage. Old or tired (too many discharge cycles) batteries probably won't. Somewhere around 5 years is typical AGM life, but it can vary a lot either way depending on how they were used and stored.

 

[Natetheskate]

The battery is 4 1/2 years old. The input charge from the panels to the controller is 20 volts in bright sunlight. The voltage from the controller is 12.7, and the battery under no load is 12.6, the last time I checked it. I haven't checked the battery immediately under extended charge by shore power, I have to check that. This doesn't bode well for an AGM battery, due to the expense. Should I return to a SLA battery?

 

[Isaac-1]

How about switching to LiFePo4 Lithium with current ever falling prices they are actually getting cheaper than AGM, even for some of the name brands, though you might have to make other changes to your electrical system, depending on how it is set up. The basic level Renogy LiFePo4 100AH battery is down to $370, LiTime's are $260, and some fly by night brands are now under $180

 

[Mark_K5LXP]

AGM's (absorbed glass mat) are a form of SLA, "sealed lead acid". What doesn't "bode well" here? Your battery voltages appear to be at rest, and normal. The question mark is your solar controller or connections between.

Lithium prices are coming down so they can compete with existing AGM applications if the charging equipment can accommodate them. If it's determined your solar controller is TU then by all means get another one that can support lithium. Then when your AGM gives up you can decide at that time what kind of battery you want to replace it with. Step one here though is to spend some quality time with a voltmeter chasing down where the fault is with what you have. It coujld be as simple as a popped fuse, failed connection or inadvertent switch setting.

Mark B.
Albuquerque, NM

 

[Natetheskate]

OK, does anyone have a recommendation for a replacement solar controller?

 

[LMHS]

I bought this one for $19.99 in July 2023. It seems to work just fine. It's no longer available. It's not a Victron or any of those other well-known brands. But I read the reviews and the people reviewing seemed happy with it. I did make sure I hooked it up in the sequence that the manufacturer/seller recommended on the selling page.